Independent labels usually specialize in one type of music (pop, rock, rap), make sure your style of music is appropriate.
Place your strongest song is first. The first verse and chorus of that song will tell them all they need to know.
Looks count. Unless you know someone in the business who'll sell your band as the next big thing, you're going to have to make a great first impression. Your press clippings, CD graphics and photo should look as if they were professionally done, even if you've created them on your home computer.

No Brainers:

• Don't send the only copy of your demo (people have actually done this.)
• Do label everything with your contact information.
• Don't ask the A&R rep. to listen to an MP3 file on your web site. Send a hard copy.
• Do type all of your submission documents (cover letter, bio, lyric sheet etc.)